1. Field of the Invention
The present invention relates to a method for forming a photo sensor in a photo diode, and more particularly, to a method for forming a photo sensor in a photo diode having better sensitivity to light of short wavelengths.
2. Description of the Prior Art
A complementary metal-oxide-semiconductor (CMOS) image sensor is a common solid-state image sensor. Since a CMOS image sensor device is produced by using conventional semiconductor techniques, the CMOS image sensor has advantages of low cost and reduced device size. In addition, the CMOS image sensor further has advantages of high quantum efficiency and low read-out noise. The CMOS image is therefore commonly used in photoelectric products, such as PC cameras and digital cameras.
A typical CMOS image sensor comprises a photo diode for sensing light, and three metal-oxide semiconductor (MOS) transistors including a reset MOS, a current source follower, and a row selector. The current in the photo diode induced by light represents a signal, whereas the current present in the device in the absence of light represents noise. The photo diode processes signal data by using the value of the signal-to-noise ratio.
Please refer to FIG. 1 . FIG. 1 is a schematic diagram of a prior art photo diode in a photo sensor located on a surface of a semiconductor wafer. As shown in FIG. 1, a semiconductor wafer 10 comprises a silicon substrate 12, a P-type well 14 located on the silicon substrate 12, a photo sensor 16 defined on the P-type well 14, and a shallow trench isolation (STI) 18 surrounding the photo sensor 16. The shallow trench isolation 18 is used to isolate the photo sensor 16 from other electrical devices for preventing an electrical shortage.
In the prior art method of forming the photo sensor 16, an ion implantation process is performed to implant N-type dopants, such as arsenic ions, into the surface of the P-type well 14 with an implantation energy of approximately 80 KeV so as to form an N-type doped region 20 with an implantation concentration of approximately 1015 cmxe2x88x922. Because the doped region 20 and the P-type well 14 are of different type dopants, a depletion region 22 is therefore formed in a PN junction between the doped region 20 and the P-type well 14. The depletion region 22 is used for sensing light.
Because dopants with a high dosage and a high energy are used in the prior art method to form the doped region 20, the width of the depletion region 22 of the PN junction formed with the doped region 20 and the P-type well 14 is narrower which leads to a smaller real active region of the photo sensor 16. This will decrease the leakage current (also called photo-current) of the PN junction when the photo-diode sensor accepts light. Therefore, the value of the signal-to-noise ratio and the photo-sensibility are reduced.
In addition, the higher implantation energy of the ion implantation process causes a deeper PN junction depth of the doped region 20. Since a short wavelength of light, such as blue light for instance, has a shallow penetration depth in the silicon wafer, the light current induced by the PN junction of the photo diode is small when the photodiode is irradiated by light of short wavelengths. Consequently, the sensitivity of the photodiode for detecting short wavelength light is reduced. Furthermore, the higher implantation energy of the ion implantation process also causes damage on the surface of the doped region 20, which leads to increasing the recombination rate of the photo charges. Therefore, the lifetime of photo charges and the sensitivity of the photo diode are decreased.
It is therefore a primary objective of the claimed invention to provide a method for forming a photo sensor in a photo diode for improving the sensitivity of a photo diode.
According to the claimed invention, a semiconductor wafer is firstly provided in the preferred embodiment of the claimed invention. The semiconductor wafer includes a substrate with a first conductive type, and an insulating layer surrounding the photo sensor. A first ion implantation process, utilizing dopants with a second conductive type, is performed to form a plurality of first doped regions in the surface of the photo sensor. A second ion implantation process, utilizing dopants with the second conductive type, is performed to form a second doped region in the surface of the photo sensor. The second doped region is overlapped with a portion of each of the first doped regions.
It is an advantage over the prior art that the claimed invention uses the first ion implantation process to form a plurality of first doped regions. Therefore, the contacting area between each first doped regions and the substrate is increased so that the sensing area of the photo diode is effectively increased. As a result, the sensitivity of the photo diode is improved. In addition, the second doped region is overlapped with the upper portion of each first doped regions, so that the second doped region and the substrate form a depletion region close to the surface of the substrate. Therefore, the sensitivity of the photo diode to light with short wavelengths (such as blue light) is effectively improved.